The invention generally relates to well testing using multiple pressure measurements.
After a well is drilled for purposes of hydrocarbon production, the well typically is tested to determine various parameters that characterize the well. For example, the well may be tested to determine the permeability of a particular formation through which the wellbore extends, as well as determining formation damage, often called the xe2x80x9cskin.xe2x80x9d
The term xe2x80x9cskinxe2x80x9d may be defined as the alteration of permeability due to fluid and particle invasion that occurs during drilling (fluid and mechanical skin respectively). In this manner, fluid and particle invasion during drilling may alter the permeability of the formation near the wellbore (called the xe2x80x9cnear wellbore formnationxe2x80x9d) and create very low permeability around the wellbore. Excessive skin may cause an excess pressure drop when the well is produced. Thus, one of the main objectives of well completion is to reduce the skin in order to improve production efficiency.
For many wells, such as horizontal wells, establishing well productivity is difficult because near wellbore formation conditions right after drilling and clean up are complex to assess. Different characteristics of the formation properties along the wellbore and their exposure to mudcake and mud filtrate for different time lengths normally creates variable skin along the wellbore that cannot be evaluated easily by using conventional well testing techniques. Furthermore, variable skin may create non-uniform-flow during production tests that hinders the interpretation of these results. Therefore, challenges to accurately assessing the skin using conventional well testing techniques exist.
Wireline techniques to assess the reservoir parameters typically produce an indication of the reservoir parameters along the near wellbore formation. Furthermore, conventional tests typically produce a single average value that characterizes the skin for the entire wellbore. Thus, a conventional test may not produce an indication of the spatial variation of the skin along a particular wellbore. However, determination of the spatial variation of the skin along the wellbore may be useful for purposes of targeting specific zones of the wellbore for cleanup and near-wellbore stimulation, as some zones may have excessive skin damage and should be isolated for purposes of treatments.
FIG. 1 depicts a typical system 10 for measuring the average skin along a wellbore 11 that extends through a formation 14. In the system 10, a tubular string 13 extends through the wellbore 11 and the annular space between the string 13 and the interior of the wellbore 11 is sealed off by a packer 12 into two isolated segments. For purposes of measuring the average skin, a flow to the surface of the well may be established through the central passageway (for example) of the tubular string 13, and in response to this flow, pressure 22 and flow 23 sensors of the string 13 may measure the respective pressure and rate of the flow. This information may be used to deduce an indication of the average skin and formation parameters associated with the whole wellbore 11 that extends through a formation 14. However, for multi-layer formation and horizontal well with long well hole, the skin and formation parameters can vary significantly. As noted above, the average skin and formation parameters of the formation 11 may not provide the enough resolution needed for proper production development and remedial work. The variations of the skin and formation parameters along well hole in the formation are needed, as average reservoir parameters are not sufficient to plan stimulation treatments or understand spatial variation of formation quality.
Thus, there exists a continuing need for an arrangement and/or technique that addresses one or more of the problems that are stated above and/or possibly addresses problems that are not stated above.
In an embodiment of the invention, a technique includes measuring the transient pressure in the wellbore at two distinct locations, which we call the first and second regions, with independent pressure sensors as the formation fluid is produced into the first region. The second region may be a passive pressure observation section.
The second region is hydraulically isolated from the first region in the wellbore, and the communication between them takes place through the formation. Formation productivity characteristics (skin, horizontal permeability or vertical permeability, as examples) are determined from the first and second measured pressures.